Giant magnetoresistance (GMR) has been observed in a variety of magnetic multilayered structures, the essential feature being at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. This GMR effect has been found in a variety of systems, such as Fe/Cr, Co/Cu, or Co/Ru multilayers exhibiting strong antiferromagnetic coupling of the ferromagnetic layers, as well as in essentially uncoupled layered structures in which the magnetization orientation in one of the two ferromagnetic layers is fixed or pinned. The physical origin is the same in all types of structures: the application of an external magnetic field causes a variation in the relative orientation of neighboring ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes.
A particularly useful application of GMR is a sandwich structure comprising two uncoupled ferromagnetic layers separated by a nonmagnetic metallic layer in which the magnetization of one of the ferromagnetic layers is pinned. The pinning may be achieved by depositing the layer onto an antiferromagnetic layer to exchange couple the two layers. This results in a spin valve magnetoresistive (SVMR) sensor in which only the unpinned or free ferromagnetic layer is free to rotate in the presence of an external magnetic field. IBM's U.S. Pat. No. 5,206,590 discloses a basic SVMR sensor for use as a read sensor or head for reading magnetically recorded data in a magnetic recording disk drive. IBM's U.S. Pat. No. 5,159,513 discloses a SVMR sensor in which at least one of the ferromagnetic layers is of cobalt or a cobalt alloy, and in which the magnetizations of the two ferromagnetic layers are maintained substantially perpendicular to each other at zero externally applied magnetic field by exchange coupling of the pinned ferromagnetic layer to an antiferromagnetic layer.
Most previously described SVMR sensors use Fe--Mn, typically Fe.sub.50 Mn.sub.50, as the antiferromagnetic layer deposited on the pinned layer for exchange coupling to fix or pin the magnetization of the pinned layer. Through exchange anisotropy with the Fe--Mn antiferromagnet, the magnetization of the pinned layer is held rigid against small field excitations, such as those that occur from the signal field to be sensed. Fe--Mn couples to nickel-iron (Ni--Fe), cobalt (Co), and iron (Fe) with an interfacial energy of 0.08 erg/cm.sup.2, and therefore is able to provide an exchange bias field in excess of the 200 Oerstead (Oe) for typical pinned layer magnetic moments. This is sufficient exchange energy to provide reasonably stable SVMR sensors. However, Fe--Mn has poor corrosion resistance.
All SVMR sensors have an interlayer exchange coupling field (H.sub.i) between the free and pinned ferromagnetic layers caused by such things as magnetostatic interactions, pin holes in the films and electronic effects. It is desirable to have a SVMR sensor with a generally low H.sub.i. SVMR sensors with the pinned layer on the bottom, i.e., on the substrate used to support the read head, require thinner Fe--Mn layers (e.g., 90 .ANG. instead of 150 .ANG.) to obtain an H.sub.i, lower than approximately 25 Oe. However, thinner Fe--Mn layers are undesirable because they have a lower blocking temperature (e.g., 130.degree. C. vs. 160.degree. C.). The blocking temperature is the temperature above which the exchange field between the Fe--Mn antiferromagnetic layer and the pinned ferromagnetic layer vanishes.
For these reasons the use of nickel-oxide (NiO) as a replacement for Fe--Mn in SVMR sensors has been proposed. The properties of SVMR sensors with NiO antiferromagnetic layers has been described by H. Hoyashi et al., Journal of the Magnetism Society of Japan, Vol. 18, p. 355 (1994); and T. C. Anthony et al., IEEE Trans. Mag., Vol. MAG-30, p. 3819 (1994). However, these results showed exchange bias fields of only about 100 Oe, which are too low for SVMR sensor applications. The NiO antiferromagnetic layers have been deposited by conventional DC or RF sputtering from a NiO target. Reactive DC or RF sputtering of a Ni target in the presence of an Ar--O.sub.2 gas to form NiO antiferromagnetic layers has been described in IBM's European patent application EP-751499, published Jan. 2, 1997, and by Shen et al., "Exchange coupling between NiO and NiFe thin films", J. Appl. Phys. 79 (8), 15 Apr. 1996, pp. 5008-5010.
The use of direction beam sputtering of a NiO target in the absence of oxygen to form an NiO antiferromagnetic layer has been described by Michel et al., "NiO Exchange Bias Layers Grown by Direct Ion Beam Sputtering of a Nickel Oxide Target", IEEE Trans. Mag., Vol. 32, No. 5, Sep. 1995, pp. 4651-4653. However, because NiO is an insulator the high energy positive ions directed at the NiO target will charge the target and ultimately prevent further ions from reaching it. Thus it is necessary to neutralize the beam by adding electrons from a hot filament wire located in the ion gun. The use of a neutralized beam complicates the SVMR sensor fabrication process because the neutralized beam is not needed to deposit the additional layers that make up the sensor and because the limited life of the filament requires the system to be shut down while the wire is replaced. In addition, the deposition rate reported by Michel et al. (0.1 .ANG./sec) would require approximately 70 minutes to deposit a 420 .ANG. layer, which is far too slow for volume production of SVMR sensors. Such a long deposition time also increases the amount of oxygen that gets distributed throughout the system, which increases the wait time needed to clear the system of oxygen before subsequent layers of the SVMR sensor can be deposited.
What is needed is a process for depositing a NiO antiferromagnetic layer that is rapid and compatible with the deposition of the other SVMR sensor layers, and which results in a SVMR sensor with good corrosion resistance, high exchange coupling between the NiO antiferromagnetic layer and the pinned ferromagnetic layer, and a low H.sub.i between the free and pinned ferromagnetic layers.